Remove Python 2 only code

[adamchainz]

• __future__.unicode_literals - no-op from Python 3.0 onwards: https://docs.python.org/3/library/__future__.html#module-contents
• coding: utf-8 comments - Python 3 defaults to UTF-8: https://docs.python.org/3/howto/unicode.html#unicode-literals-in-python-source-code

[Gagaro]

Thanks for the PR :+1: